1. Field of the Invention
The present invention relates generally to systems and methods for recording data on optical storage media. In particular, the invention relates to a system for focusing a light beam on the optical recording media to deform a region within an expandable layer of the media.
2. Description of the Background Art
Optical data storage media in the form of compact disks are well known-as an alternative to long playing records and magnetic tape cassettes. The disks with which consumers are familiar are optical read-only disks and the common disk player is designed specifically for this type of disk. These disks have a reflective surface containing pits which represent data in binary form. A description of these pits and how they function is provided by Watkinson, "The Art of Digital Audio," Focal Press, Chapter 13.
Compact disks are currently produced by a pressing process similar to the process used to produce conventional long playing records. The process, referred to herein as the "mastering" process, starts by first polishing a plain glass optical disk. This disk has an outside diameter from 200 to 240 mm, a thickness of 6 mm and undergoes various cleaning and washing steps. The disk is then coated with a thin chrome film or coupling agent, a step taken to produce adhesion between the glass disk and a layer of photo-resist, which is a photosensitive material. Data on a compact disk master tape are then transferred to the glass disk by a laser beam cutting method.
The glass disk is still completely flat after it is written on by the laser beam because pits are not formed until the glass is photographically developed. The disk surface is first made electrically conductive and then subjected to a nickel evaporation process. The disk, now known as the glass master, then undergoes nickel electrocasting, a process which is similar to that used in making analog phono records. A series of metal replications follow, resulting in a disk called a stamper. The stamper is equivalent to a photographic negative in the sense that it is a reverse of the final compact disk; that is, there are now bumps where there should be pits. This stamper is then used to make a pressing on a transparent polymer such as polyvinyl chloride, poly(ethyl-metacrylate) and polycarbonate. The stamped surface is then plated with a reflective film such as aluminum or other metal and finally a plastic coating is applied over the film to form a rigid structure.
The player operates by focusing a laser beam on the reflective metal through the substrate and then detecting reflected light. The optical properties of the substrate, such as its thickness and index of refraction, are thus critical to the player's detection systems and standard players are designed specifically with these parameters in mind.
The pits increase the optical path of the laser beam by an amount equivalent to a half wavelength, thereby producing destructive interference when combined with other (non-shifted) reflected beams. The presence of data thus takes the form of a drop in intensity of the reflected light. The detection system on a standard player is thus designed to require greater than 70% reflection when no destructive interference occurs and a modulation amplitude greater than 30% when data is present. These intensity limits, combined with the focusing parameters, set the criteria for the compact disks and other optical data storage media which can be read or played on such players.
Media on which data can be recorded and read have a different configuration and operate under a somewhat different principle. One example is described in U.S. Pat. No. 4,719,615 (Feyrer et. al.), the disclosure of which is incorporated herein by reference. A second relevant patent is U.S. Pat. No. 4,879,709, the disclosure of which is incorporated herein by reference. Other examples are described in copending U.S. patent application Ser. Nos. 294,723; 357,377; 357,504; 357,506; 414,044; and 414,041, the disclosures of which are incorporated herein by reference.
The medium disclosed in Feyrer et. al, includes a lower expansion layer of a rubbery material which expands when heated. The expansion layer is coupled to an upper retention layer which is glassy at ambient temperature and becomes rubbery when heated. Both layers are supported on a rigid substrate. The expansion and retention layers each contain dyes for absorption of light at different wavelengths. Data are recorded by heating the expansion layer by absorption of light from a laser beam at a "record" wavelength to cause the expansion layer to expand away from the substrate and form a protrusion or "bump" extending into the retention layer. While this is occurring, the retention layer rises in temperature above its glass transition temperature so that it can deform to accommodate the bump. The beam is then turned off and the retention layer cools quickly to its glassy state before the bump levels out, thereby fixing the bump. Reading or playback of the data is then achieved by a low intensity "read" beam which is focused on the partially reflecting interface between the retention layer and air. When the read beam encounters the bump, some of the reflected light is scattered, while other portions of the reflected light destructively interfere with reflected light from non-bump areas. The resulting drop in intensity is registered by the detector. Removal of the bump to erase the data is achieved by a second laser beam at an "erase" wavelength which is absorbed by the retention layer and not the expansion layer. This beam heats the retention layer alone to a rubbery state where its viscoelastic forces and those of the expansion layer return it to its original flat configuration. The write, read and erase beams all enter the medium on the retention layer side, passing through retention layer before reaching the expansion layer.
The size and shape of the bump or protrusion formed on the optical storage media described in Feyrer et al. or in any of the pending patent applications depend directly on the intensity and distribution of energy within the light spot which is formed on the surface of the media. Only light having an intensity greater than a threshold intensity level (dependent primarily on the material of the expansion layer) will result in expansion of the material in the expansion layer. Thus, the geometry of the bump which is formed by the light spot depends directly on the projected area of light having an intensity above the threshold level.
In order to achieve a maximum data density on the recording media, the projected spot size having an intensity above the threshold level must be as small as possible. In previous recording systems, the laser light source generally has a Gaussian energy distribution and is focused to a spot on the recording media by a high numerical aperture objective lens, resulting in a projected spot having a similar Gaussian energy distribution. Thus, the projected area of light having an intensity above the threshold level will have an effective diameter which is inversely proportional to the diameter of the incident beam on the objective lens and directly proportional to the focal length of the lens and the wavelength of the light. The minimum effective spot diameter is thus limited by these three system parameters so long as a normal objective lens is employed.
For these reasons, it would be desirable to provide improved methods and systems for projecting light beams onto optical recording media where the effective diameter of the spot of light incident on the recording media is reduced in order to increase the available data density. In particular, it would be desirable to modify the intensity profile of the light spot to minimize the projected area having an intensity greater than the threshold intensity. It would be particularly desirable to achieve such a spot size reduction without substantial modification of existing system designs, such as by using existing laser light sources and objective lenses.